1. Homeostasis and feedback The Human Body
An IRSC Live Virtual Lesson
By: Diana Lenartiene, Ed. S.

2. Definition of Homeostasis
The biological definition of homeostasis is “the tendency of an organism or cell to regulate its internal environment and maintain equilibrium, usually by a system of feedback controls, so as to stabilize health and functioning”. Generally, the body is in homeostasis when it’s needs are met and it’s functioning properly.

3. Every organ in the body contributes to homeostasis
Every organ in the body contributes to homeostasis. A complex set of chemical, thermal, and neural factors interact in complex ways, both helping and hindering the body while it works to maintain homeostasis.

4. Homeostatic control
To maintain homeostasis, communication within the body is essential. The image below is an example of how a homeostatic control system works. Here is a brief explanation:
Stimulus- produces a change to a variable (the factor being regulated).
Receptor- detects the change. The receptor monitors the environment and responds to change (stimuli).
Input- information travels along the (afferent) pathway to the control center. The control center determines the appropriate response and course of action.
Output- information sent from the control center travels down the (efferent) pathway to the effector.
Response- a response from the effector balances out the original stimulus to maintain homeostasis.

5. Think of it as an extremely complex balancing act
Think of it as an extremely complex balancing act. Here’s a diagram of the process.

6. Explaining homeostasis
We will now view a video
Explaining homeostasis
8 minute video marked use first

7. Afferent pathways- carry nerve impulses into the central nervous system. For instance, if you felt scorching heat on your hand, the message would travel through afferent pathways to your central nervous system.
Efferent pathways- carry nerve impulses away from the central nervous system to effectors (muscles, glands).
The feeling of heat would travel through an afferent pathway to the central nervous system. It would then interact with the effector and travel down the efferent pathway, eventually making the person remove their hand from the scorching heat.

8. Negative feedback mechanisms
Almost all homeostatic control mechanisms are negative feedback mechanisms. These mechanisms change the variable back to its original state or “ideal value”.
A good example of a negative feedback mechanism is a home thermostat (heating system). The thermostat contains the receptor (thermometer) and control center. If the heating system is set at 70 degrees Fahrenheit, the heat (effector) is turned on if the temperature drops below 70 degrees Fahrenheit. After the heater heats the house to 70 degrees Fahrenheit, it shuts off effectively maintaining the ideal temperature.

9. Stimulus and response flowchart of feedback

10. The control of blood sugar (glucose) by insulin is another good example of a negative feedback mechanism. When blood sugar rises, receptors in the body sense a change . In turn, the control center (pancreas) secretes insulin into the blood effectively lowering blood sugar levels. Once blood sugar levels reach homeostasis, the pancreas stops releasing insulin.
These are just two examples of negative feedback mechanisms within our body, there are 100’s, can you think of a few more?

11. Positive feedback mechanisms A positive feedback mechanism is the exact opposite of a negative feedback mechanism. With negative feedback, the output reduces the original effect of the stimulus. In a positive feedback system, the output enhances the original stimulus. A good example of a positive feedback system is child birth.
During labor, a hormone called oxytocin is released that intensifies and speeds up contractions. The increase in contractions causes more oxytocin to be released and the cycle goes on until the baby is born. The birth ends the release of oxytocin and ends the positive feedback mechanism.

12. Another good example of a positive feedback mechanism is blood clotting. Once a vessel is damaged, platelets start to cling to the injured site and release chemicals that attract more platelets. The platelets continue to pile up and release chemicals until a clot is formed.
Just remember that positive feedback mechanisms enhance the original stimulus and negative feedback mechanisms inhibit it.

13. Compare negative and Positive feedback
Negative Feedback
Positive Feedback
1.Resist change in the body
and bring back the original state.For example when you are exposed to cold your body temperature drops and your body make changes to bring it back to the body temperature. 2. In this feedback loop, the values remain with in a range. For example blood sugar is maintained at 100mg/ml. If blood sugar level goes up te control center pancreas steps in and produce insulin hormone, which helps to pump sugar into cells.Once the sugar levels reach normal pancreas stops secreting insulin.
3. Common in the body
 4. This feedback loop is initiated by a stimulus that disturbs the homeostasis of a body system.For e.g. Eating sugar disrupts the glucose balance in the blood and triggers the Negative feedback loop of insulin.
1. Reinforce change
in the body that results in a positive out come. For example when a pregnant mother is ready to deliver the bab, the hormone oxytocin levels shoots up causing stronger uterine contractions. 2. Values goes out of range
3. Very uncommon
 4. Positive feedback loop is also triggered by a stimulus and

14. Table 1: Types of Homeostatic Regulation in the Body
Homeostatic Processes
Hormones and Other Messengers
Tissues, Organs and Organ Systems Involved
Osmoregulation (also known as excretions)
Excess water, salts, and urea expelled from body
Antidiuretic hormone (ADH), aldosterone, angiotensin II, carbon dioxide
Kidneys, urinary bladder, ureters, urethra (urinary system), pituitary gland (endocrine system), lungs (respiratory system)
Thermoregulation
Sweating, shivering, dilation/constriction of blood vessels at skin surface, insulation by adipose tissue, breakdown of adipose tissue to produce heat
Nerve Impulses
Skeletal muscle (muscular system), nerves (nervous system), blood vessels (cardiovascular system), skin and adipose tissue (integumentary system), hypothalamus (endocrine system)
Chemical Regulation (including glucoregulation)
Release of insulin and glucagon into the blood in response to rising and falling blood glucose levels, respectively; increase in breathing rate in response to increases carbon dioxide levels in the blood, and release of carbon dioxide into exhaled air from lungs, secretion of erythropoietin by kidneys to stimulate formation of red blood cells
Insulin, glucagon, cortisol, carbon dioxide, nerve impulses, erythropoietin (EPO)
Pancreas (endocrine system), liver (digestive system); adrenal glands (endocrine system) lungs (respiratory system), brain (nervous system), kidneys (urinary system)

15. We will now view a video on Homeostasis and feedback
18 minute video explanation.

16. What we have learned
Homeostasis is from the Greek and means staying the same.
Homeostasis has internal and external stimuli which effect it and cause the body to give negative or positive feedback.
Negative feedback is more common than positive feedback
A feedback loop is the set of stimulus and response activities related to a specific situation. We studied several examples.
Prolonged exposure to negative feedback can cause disease or death. This is why Homeostasis is so important to the human body.

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Diana Lenartiene, IRSC ABE Instructor